PV is a technology typically using large area p-n junction diodes to convert sunlight into electricity. These p-n junction diodes are therefore called solar cells. When a solar cell is exposed to the solar irradiance, photons from the sunlight that have energy greater than the band gap of the semiconductor material of which a cell is made create electron-hole pairs in the cell. The asymmetrical characteristics of the p-n junction determine the flow direction of the photo-generated carriers of different types contributing to the cell output that can be extracted from the cell's terminals, in a way similar to a normal electrochemical battery.
FIG. 1 illustrates a basic single p-n junction solar cell 100 made from a p-type substrate. Referring to FIG. 1, the solar cell 100 includes front contact fingers 102, an antireflection (AR) layer 104, an n+ layer 106, a p-type mono Si substrate 108, a back-side-field (BSF) p+ layer 110, and a rear metal contact 112. The n+ layer 106 covers the top or front surface of the substrate 108. The antireflection layer 104, which may be made of silicon nitride (SiNx), covers the entire surface of the n+ layer 106, and the front contact fingers 102 are spatially embedded in the antireflection layer 104. On the back side of the substrate 108, the p+ layer 110 covers the back surface of the substrate 108, and the back metal contact 112 covers the entire surface of the p+ layer 110. The antireflection layer 104 and the n+ layer 106 have a triangular wave shape to reduce reflection loss.
The theoretical energy conversion efficiency of a PV solar cell consisting of one p-n junction like the solar cell 100 in FIG. 1 is about 31%, which is the electricity power that can be extracted from the cell relative to the total solar irradiance into the cell. Solar cells can be made from various materials, but solar cells made from mono-Si are the most efficient in energy conversion, excluding the expensive compound semiconductor cells which are almost explicitly used for extraterrestrial applications. Mono-Si solar cells are generally fabricated on p-type Si substrates cutting from boron-doped (B-doped) single crystal ingots mainly produced by Czochralski (CZ) growth method. The CZ method is a much preferred method over another so-called floating-zone (FZ) method in the solar industry because of its capability of producing inexpensive, large-diameter single-crystal ingots with excellent strength. The energy conversion efficiency of mono-Si solar cells made from B-doped p-type substrates in massive production is typically limited to around 16-17% due to various loss mechanisms. One loss mechanism is due to the recombination of the photo-generated carriers caused by defects and impurities that adversely shorten the lifetime of the minority carriers in the p-type absorber region, in addition to the non-radiative recombination in he heavily doped n-type region and at its surface.
It is known that the lifetime of minority carriers (electrons) decreases drastically in B-doped p-type mono-Si wafers when the concentration of B dopants is slightly below or comparable to that of the background oxygen impurity resulting from the dissolution of silica crucible during CZ crystal growth. Solar cells made from the B-doped substrates generally exhibit a certain degree of degradation after subjecting to strong photo-illumination presumably caused by high concentration oxygen impurity interacting with B dopants, shortening the lifetime of electrons and therefore limiting the conversion efficiency.
Using gallium (Ga) doped p-type substrates is one way to overcome the problem. Ga-doped Si wafers are observed to have a much longer lifetime of minority carriers without showing photo-degradation effect. Fabrication using Ga-doped substrates in the same resistivity range as B-doped wafers yields solar cells consistently showing higher conversion efficiency compared to those using B-doped substrates. However, it is well known that Ga-doped single-crystal Si ingots grown by CZ method have very poor doping distribution uniformity in both axial and radial directions primarily due to the very small equilibrium segregation coefficient for Ga dopant in Si. Shown in FIG. 2 is a comparison of doping distributions as a function of the fraction of solidified melt between Ga and B dopants, where Co and Cs are equilibrium concentrations of dopants in the solid and the liquidified melt, and the equilibrium segregation coefficients are 0.008 and 0.8 for Ga and B, respectively. The small equilibrium segregation coefficient for Ga dopant in Si crystal growth inherently poses a great challenge to the industry as how to obtain a uniform doping distribution throughout the entire CZ growth process. Even with controlling pull rate and rotation speed during the growth, large variation in resistivity from wafer to wafer resulting from non-uniform Ga doping distribution in Si ingots is still inevitable.
Recombination in the heavily doped diffusion region and at its surface is another major loss channel that impacts the conversion efficiency of mono-Si solar cells. In general, a more heavily doped region results in greater recombination since there is more Auger recombination present in the region. Addition of a surface passivation film, such as oxide, usually reduces the recombination at the surface, only partially resolving the problem.
Another loss mechanism in energy conversation efficiency is the shadowing effect of the front contact fingers. The shadowing effect of front contact fingers alone may cause a reduction in current density resulting from the coverage of a metal grid of the fingers on the front face of a cell that blocks about 4˜5% of the sunlight incident onto the cell surface. To overcome this shadowing problem, Lammert and Schwartz proposed an interdigitated back contact structure thirty years ago (M. D. Lammert and R. J. Schwartz, IEEE Trans. Electron. Dev. 31, 337 (1977)). By placing both n+ and p+ diffused contact regions on the back of a cell, the shadowing effect can be avoided. The series resistance can be further lowered as well as a result that the metal contacts may take up almost entire back surface. So far, full backside contacted solar cells have only been successfully developed using n-type mono-Si. Using p-type mono-Si wafers to fabricate full backside contacted solar cells should be, in principle, more advantageous because the diffusion length of minority carriers in a p-type Si wafer is much longer. Unfortunately, the vastly available and the most used CZ grown B-doped p-type Si single crystal wafers were found to consistently exhibit a not-so-well-understood degrading behavior in the lifetime of minority carriers (electrons) after subjecting to strong photo-illumination, preventing cells fabricated using B-doped wafers from achieving high conversion efficiency. The consensus to the problem is that the phenomenon is most likely to be caused by the interaction of B dopants with oxygen impurity that were incorporated into the Si crystal during CZ growth, presumably forming B-O complex, resulting in the degradation of the effective lifetime of minority carriers and therefore limiting the cells' conversion efficiency. On the other hand, Ga-doped p-type mono-Si wafers do not have the problem. Much longer lifetime of minority carriers were observed in Ga-doped Si wafers with no photo-degradation effect. Solar cells fabricated using Ga-doped wafers in the same resistivity range as B-doped wafers have consistently shown to be of higher conversion efficiency in comparison with those using B-doped substrates. With the technology of CZ growth of Ga-doped Si being rapidly advanced, fabricating fully backside contacted solar cells using p-type Si wafers has become a reality in terms of achieving better cells with higher conversion efficiency and using less amount of expensive Si material per cell for its thinner cell structure.